The complexities of natural product synthesis and of the rapidly developing field of carbohydrate synthesis create a demand for chemically differentiable protecting groups (PGs) for vulnerable functionality. Benzyl ethers are among the most popular alcohol Pgs due to their ease of formation, stability to a wide range reaction conditions, and mild cleavage protocols. Modified arylmethyl PGs have been tailored for use in more complex systems.
Several arylmethyl PGs are cleaved by initial transformation into a para-hydroxbenzyl (PHB) ether. Jobron and Hindsgaul first reported the use of O-protected 4-O-benzyl PGs for carbohydrate chemistry. Removal of the arene 4-O-PG under the appropriate conditions reveals the PHB ether, which is then easily hydrolyzed. Cross-coupling of para-bromobenzyl (PBB) ethers provides a similar effect: palladium-catalyzed amination of the PBB group yields a labile para-aminobenzyl ether, whereas palladium-catalyzed borylation of PBB followed by oxidation afforded a PHB ether in a synthetic approach to ciguatoxin.
These past efforts reflect the importance of diverse arylmethyl PGs and highlight the need for orthogonality and functional group compatibility in the cleavage event.
As noted above, benzyl ethers are among the most common and important protecting groups in organic synthesis. Like other alkyl ethers, they are advantageous for their stability to a wide range of reaction conditions and for the minimal electronic impact that they impart on the oxygen atom to which they are attached. For example, benzyl ethers are often employed to establish chelation control during addition to chiral aldehydes, which provides selectivity opposite that predicted by the acyclic Felkin-Anh model and observed with bulky silyl ethers. Similarly, benzyl-protected glycosyl donors are “armed” relative to acylated analogues. Among alkyl ethers, benzyl (and modified arylmethyl)ethers are perhaps the most versatile with respect to modes of cleavage, which include hydrogenolysis, oxidation, and acidic decomposition under a range of experimental protocols.
Relatively harsh conditions are typically required for generating benzyl ethers from the corresponding alcohol, with the two most popular protocols being (1) the Williamson ether synthesis, an SN2-type reaction between alkali metal alkoxides and benzyl bromide, and (2) coupling using benzyl trichloroacetimidate, which is generally promoted by trifluoromethanesulfonic acid (triflic acid, TfOH). Typical benzylation reactions are thus limited to substrates that tolerate either strongly acidic or basic conditions. β-Hydroxy esters, for example, are subject to several acid- or base-catalyzed reactions, including retro-Aldol, elimination, and epimerization of stereogenic centers—to the carbonyl group. Benzylation of these ubiquitous intermediates in the synthesis of polyketides and other important compounds can be problematic. Selective protection of polyol systems (e.g., carbohydrates) can also be complicated by base-catalyzed migration of esters and silyl ethers and by acid-catalyzed cleavage of silyl ethers and acetal linkages.
A recent review addresses the myriad options for protecting alcohols using mild, convenient, and environmentally friendly conditions, but no methods for the formation of benzyl ethers are discussed. Silylation and acylation of alcohols can be accomplished under effectively neutral conditions using activated reagents that react with the free alcohol. Imidazole and DMAP are frequently employed to activate silyl and acyl chlorides; conveniently, they are also capable of scavenging any acid that is produced during the course of the reaction. Protonation of benzyl trichloroacetimidate provides an activated reagent that reacts with free alcohols, but this mode of activation precludes neutralization of free acid. In principle, covalent activation (alkylation) of a trichloroacetimidate surrogate would enable the formation of benzyl ethers in the absence of external base or acid and in the presence of acid scavengers (if desired).
Accordingly, benzylation of alcohols under mild and nearly neutral conditions, as disclosed herein, would constitute a significant advance in synthetic chemistry. Thus, it was envisioned that 2-benzyloxypyridine could serve as an imidate surrogate for benzylation of alcohols. Pyridinium salts have been employed in esterification reactions, with Mukaiyama's 2-chloro-1-methylpyridinium iodide being perhaps the most popular.
Conversion of alcohols into thioesters and azides using 2-fluoro-1-methylpyridinium tosylate has also been demonstrated. The two pieces of prior knowledge that were most influential in guiding the current work are: (1) certain 2-alkoxypyridinium bromides decompose to bromoalkanes and pyridones; and (2) 2-alkoxypyridinium sulfonates do not proceed spontaneously to alkyl sulfonates. It was hypothesized that decomposition of 2-alkoxypyridinium sulfonates in the presence of alcohols would give rise to alkyl ethers and pyridones, and preliminary data in support of this hypothesis was reported from this laboratory.